1. Field of the Invention
The present invention relates to a control device of a permanent magnet synchronous motor for preventing irreversible demagnetization of a permanent magnet of a permanent magnet synchronous motor, and a control system including such a control device.
2. Description of Related Art
When controlling a permanent magnet synchronous motor, there is a case where it is no longer possible for the permanent magnet synchronous motor to generate a torque quickly and stably due to irreversible demagnetization caused by a rise in permanent magnet temperature of a permanent magnet of the permanent magnet synchronous motor. Because of this, various kinds of motor control devices for preventing irreversible demagnetization of the permanent magnet of the permanent magnet synchronous motor are well known.
For example, as a conventional motor control device for preventing irreversible demagnetization of a permanent magnet of a permanent magnet synchronous motor, a motor control device configured to estimate a permanent magnet temperature, predict a short circuit current value in the case where a failure of a switching element of a power converter occurs at the estimated permanent magnet temperature, and limit the maximum speed of a permanent magnet synchronous motor so that the predicted short circuit current value is equal to or less than a current value at which irreversible demagnetization occurs in the permanent magnet (irreversible demagnetization causing current value), is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2009-5553 (JP2009-5553A).
Further, as a conventional motor control device for preventing irreversible demagnetization of a permanent magnet of a permanent magnet synchronous motor, a motor control device for preventing occurrence of irreversible demagnetization by increasing the carrier frequency of a power converter or by changing the modulation scheme inside the power converter from the two-phase one to the three-phase one in order to reduce the permanent magnet temperature in the case where there is a possibility that irreversible demagnetization occurs while the permanent magnet synchronous motor is being driven, is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2006-254521 (JP2006-254521A).
Furthermore, as a conventional motor control device for preventing irreversible demagnetization of a permanent magnet of a permanent magnet synchronous motor, a control device of a permanent magnet synchronous motor configured to adjust the output of a permanent magnet synchronous motor in accordance with the permanent magnet temperature of a permanent magnet of the permanent magnet synchronous motor, is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-18496 (JP11-18496A) and Japanese Unexamined Patent Publication (Kokai) No. 2003-235286 (JP2003-235286).
Because of its inherent characteristics, the permanent magnet synchronous motor may operate as a power generator at emergency of the permanent magnet synchronous motor (for example, at the time of power failure of the power source configured to supply power to the permanent magnet synchronous motor). When the permanent magnet synchronous motor operates as a generator, there is a case where a problem, such as burn-out, occurs due to excessive voltage produced in the permanent magnet synchronous motor itself and a motor control device for controlling the permanent magnet synchronous motor. As measures against such a problem, normally, the three phases of the power lines of the permanent magnet synchronous motor are short-circuited by a short-circuiting device when the permanent magnet synchronous motor is abnormal, thereby the permanent magnet synchronous motor is stopped safely.
In the case where the three phases of the power lines of the permanent magnet synchronous motor are short-circuited by the short-circuiting device at emergency of the permanent magnet synchronous motor, a transient current is produced. The transient current produced in such a manner flows in the direction in which the permanent magnet of the permanent magnet synchronous motor is demagnetized, and therefore, there is a possibility that irreversible demagnetization occurs in the permanent magnet of the permanent magnet synchronous motor.
In the conventional motor control devices described above, no measures are taken against the transient produced in the case where the three phases of the power lines of the permanent magnet synchronous motor are short-circuited, and therefore, it is not possible to prevent irreversible demagnetization of the permanent magnet of the permanent magnet synchronous motor caused by the transient current produced in the case where the three phases of the power lines of the permanent magnet synchronous motor are short-circuited.